(1) Field of the Invention
The present invention relates to an apparatus for selecting dedicated control channels in a mobile communication system in a service area having two competing service systems.
(2) Description of the Related Art
The automobile has become indispensable in contemporary life, and comfort and convenience when driving has become increasingly important. To this end, vehicles can be provided with a mobile communication system which can be used as a telephone system.
An automobile equipped with a mobile communication system, e.g., a mobile telephone system, is called a mobile station, and this mobile station can communicate not only with a fixed telephone such as a telephone in an office or at home but also with another mobile station. This communication is established by radio waves between the mobile station and a land station, and transferred to a conventional telephone system at the land station.
In a typical mobile communication system, for example, the "Cellular system" used in the United States, two companies may compete in one service area, i.e. a system A and a system B exist in one service area as shown in FIG. 1. Each system has a certain number of control channels and a large number of voice channels having different frequency ranges. For example, in the Cellular system, there are 21 control channels and 312 voice channels for each system, as shown in FIG. 2. The frequencies .alpha. to .beta. are assigned to the voice channels of system A, and the frequencies .beta. to .gamma. are assigned to the control channels of system A. The frequencies .gamma. to .delta. are assigned to the control channels of system B, and the frequencies .delta. to .epsilon. are assigned to the voice channels of system B (.alpha.&lt;.beta.&lt;.gamma.&lt;.delta.&lt;.epsilon.). Each mobile station subscribes to one of the two systems, A or B; in the Cellular system, the subscribed-to system is called the "home system" and the other system is called the "roam system".
Note, since these terms "home system" and "roam system" in the Cellular system enable an easy understanding and discrimination between the subscribed-to system and the other system, these terms will be used hereinafter.
When power is applied to the mobile station, the mobile state first scans the dedicated control channels of the home system, and tunes in to the strongest or second strongest dedicated control channel of the home system. In other words, the mobile station seeks a dedicated control channel on which an outgoing call (call send) or incoming call (call receive) can be reliably transmitted or received. After the mobile station is tuned to one of the dedicated control channels, it enters a call send or call receive waiting state. In this state, if a call send or call receive mode is activated by the user, the mobile station tunes in to one of the voice channels in accordance with a command from the land station through the dedicated control channel. The scan of the dedicated control channel is activated at predetermined periods, such as every 5 minutes, or after the end of each call.
If the mobile station cannot tune in to the strongest or second strongest dedicated control channel during the scan of the home system, then it is determined that the home system cannot be used, and thus the mobile station scans the dedicated control channels of the roam system. If during this scan, the mobile station can tune in to the strongest or second strongest dedicated control channel of the roam system, the mobile station enters a call send or call receive waiting state in the roam system. Namely, the mobile station can use the roam system when the home system is not available. However, if a call send or call receive mode is activated when the mobile station is in the waiting state in the roam system, the mobile station must pay an extra fee to the roam system.
The mobile station may move through an area which is a blind section CA of the home system as shown in FIG. 1. For example, the blind section CA can be due to a high building or a mountain between the mobile station and the land station, and is not necessarily a blind section for the roam system. Therefore if the mobile station scans the dedicated control channels at this time, the mobile station enters the call send or call receive waiting state in the roam a system, and this state cannot be changed until the next scan of the dedicated control channel, even if the mobile station has passed out of the blind section CA in the meanwhile. If a call send or call receive mode is activated in the above state, the mobile station must use the roam system even though the mobile station is in an area where the home system is available, and thus must pay an extra fee.